Optical scopes are mounted on rifles to provide a clear magnified image of a target and to provide an adjustable aiming point to indicate the point of impact of a projectile on a target. Scopes typically have an internal optical adjustment to shift the image or apparent location of the crosshairs or other reticle to compensate for the amount that the bullet drops below the axis of the barrel as it travels over a distance. A similar lateral adjustment is normally provided for windage compensation.
On firearms having a relatively short maximum effective range (under about 600 meters), the scope may be mounted such that its sighting axis is approximately aligned with the bore axis of the barrel and is adjusted vertically in elevation and adjusted laterally in windage such that the point of aim observed by the shooter is the point of impact of the projectile at the desired range. Other elevation and windage adjustments may be necessary based on number of well known factors including wind speed and direction, temperature, humidity, projectile shape and mass, and powder mass and burn characteristics. Since projectiles follow a ballistic path, adjustments of elevation may be a critical factor for hitting targets at ranges approaching the maximum range of the cartridge-rifle combination. Because all projectiles are affected by the Earth's gravitational pull, the ballistic path always follows a downward arc, requiring the bore axis of the barrel to be elevated relative to the target (or for the line of sight to be declined in elevation relative to the bore axis of the barrel).
The range in elevation adjustments needed for telescopic scopes mounted to high powered sporting and military rifles capable of hitting targets at distances greater than about 600 meters frequently exceeds the range in elevation adjustments achievable by adjustment mechanisms incorporated within the telescopic scope itself. Scope-mounting systems are available that provide a selected angular deviation that points the scope slightly downward with respect to the barrel bore axis. The angular deviation is typically less than one degree, with a deviation in the range of 10-50 minutes of angle (MOA) being typical. The deviation may be manufactured into a rail system to which a scope mount is attached or in a scope mount having rings encompassing a scope tube, with the rear ring at a higher elevation than the front ring. This allows use of a practical internal scope adjustment mechanism that employs the upper range of adjustment for nearer shots and the lower range below a neutral setting for more distant shots where bullet drop is greatest.
While a mount having a single, preselected angle of declination is effective for use with a given cartridge or firearm to hit targets within a selected range of distances, a single angled mount is not suitable for a varied range of cartridges or versatile for a wider range of target distances. Thus, numerous different-angled mounts must be manufactured and stocked, and users must buy multiple mounts to provide for different applications or settle for a compromise that is suboptimal at other extreme of the range. Changing the mount may be inconvenient for a sporting shooter who uses the same rifle to compete at 500 meters and 1,500 meters. But, for a sniper operating in a theater of war who may engage targets at as little as 350 meters and at well over 1,500 meters during the same mission, changing the mount is impractical, if not impossible.
In the past, field-adjustable mounts have been proposed. One type provides a pivot axis for moving the angle of a mounting rail. Another uses a pivot in one of the scope mounting rings with the other ring being adjustable in height. When sighting accuracy within ¼ MOA (or a 0.1 milliradian) is necessary and such mounts are subjected to the repeated recoil impact forces of a heavy caliber rifle, a pivotal mounting system may not provide sufficient stability and durability.